Environmentally assisted cracking (EAC), which encompasses stress corrosion cracking and sulphide stress cracking, is a commonly observed phenomenon that results in the premature failure of metals. EAC is typically caused by the exposure of a sensitive metal to a corrosive environment and stress. If the corrosive environment or stress is absent, the metal will not crack. Stress, can be either residual, for example, from manufacturing, or applied, due to operations or improper handling.
Environmentally assisted cracking has caused severe structural failures over a broad range of industrial applications. This problem is particularly severe in the oil and gas industry, which has experienced a significant increase in EAC failures of production tubing or pipelines. These failures have predominantly occurred with martensitic and duplex stainless steel tubing with the cracks generally emanating from the annular side of the production tubing. This phenomenon is known as annular environmentally assisted cracking (AEAC). Failures of metal pipes have resulted in multi-million dollar expenses due to lost production time, replacement of production tubing and increased manpower and rig time utilization, among other factors. The prevention, prediction and control of EAC have assumed greater significance in recent years because of the increasing incidence of downhole tubing failures attributed to EAC. While various factors influence cracking, in most of these cases, cracking begins from the tubing's outer surface, rather than the inside.
The location of these cracks has led corrosion scientists to posit that the cracks are a result of corrosive packer fluids that interact with the metal tubing. However, there are no guidelines for the selection of fluids that are compatible with the various metals. As a consequence, the selection is made with limited information available from published literature or individual laboratory tests or is made based on pure conjecture due to lack of information.
Laboratory testing is typically conducted in accordance with NACE guidelines, wherein metals are subject to stress levels limited to the elastic region. These tests frequently involve non-representative fluids and test conditions that are not representative of those encountered in oil field applications. The duration of these tests may also be too long to be practical for the accumulation of a meaningful volume of test data, with test durations ranging from 14 days to 30 days for a standard test. The most common tendency, where the test data is lacking or is non-conclusive, is to select a relatively more expensive oil field fluid in order to minimize the risks of EAC and AEAC.
Previous selection of metals used in the oil and gas industry was done without substantive information on AEAC and the compatibility of various fluids with the common corrosion resistant metals used in production tubing. Particularly unfortunate has been the reliance on NACE methodologies, which involve non-representative fluids and well conditions, thereby leading to erroneous conclusions.
Consequently, in order to minimize the risk of metal tubing failures and to improve the economics of selecting compatible oil field fluids, there exists a need for a system that allows for a quick determination of these fluids that are compatible with the metals under corrosive oil field conditions.